Generate correct number of reads with SimNGS

Added a trimming function that remove the excess of reads that SimNGS generates when the target organism has more than one chromosome. 

Here is a sample after running the tests:
```
(master)guillem@guillemmac:~/repos/facs/tests/data/synthetic_fastq (master)$ for f in simngs*; do wc -l $f; done
   36000 simngs.mixed_eschColi_K12_dm3_3000vs6000.fastq
     400 simngs_dm3_100.fastq
    4000 simngs_dm3_1000.fastq
 4000000 simngs_dm3_1000000.fastq
 40000000 simngs_dm3_10000000.fastq
     400 simngs_eschColi_K12_100.fastq
    4000 simngs_eschColi_K12_1000.fastq
 4000000 simngs_eschColi_K12_1000000.fastq
```